The invention is related to the field scanning microscopy, in particular a novel technique for high resolution scanning optical microscopy that uses an absorbance-modulation layer on top of a sample or substrate to generate optical near-fields from the far-field.
The conventional technique in this field is near-field scanning microscopy. In this technique a physical probe is brought into close proximity of a sample. The optical near-field that emanates from the probe interacts with the sample. Detection of scattered light followed by signal processing reveals information at high resolution, for example, beyond the far-field diffraction limit. There are many problems associated with using the conventional technique. One problem is the close proximity of a probe and a sample increases the likelihood of damaging the sample. Since the signal is very sensitive to the gap between the probe and the sample, a complex feedback-based control system is required to maintain this constant gap. Most importantly, this technique is very slow because it is a serial process. It is extremely difficult to parallelize this technique using multiple probes due to the difficulties associated with maintaining the nanoscale gap for each probe accurately.
More recently, an alternative technique that utilizes reversible saturable transitions was proposed. In this technique, first, an excitation (for example of fluorescence) is carried out by a focused laser beam. Then, the excitation is quenched by a second laser beam that is focused to a ring-shaped spot. The excitation is quenched everywhere except at a small region near the center of this ring-shaped spot. The signal from this small region is then collected. By scanning the sample with respect to the optics, an image is built up. However, the material (or molecule) that undergoes reversible saturable transitions is intimately associated (for example, chemically) with the sample. Also, the excitation is diffraction limited (and hence, large), while the signal is localized via quenching. This limits the type of signals that may be studied.